In general, a thermally sensitive recording medium is prepared by following process. That is, after a colorless or pale colored basic colorless dye and an organic color developing agent are respectively ground to fine particles, these particles are mixed together with and additives such as a binder, a filler, a sensitizer and a slipping agent or others are added and obtain a coating, then the obtained coating is coated on a substrate such as a paper, a synthetic paper, a film or a plastic. The thermally sensitive recording medium develops color by instant chemical reaction caused by heating with a thermal head, a hot stamp, a thermal pen or laser beam. Usually, a thermally sensitive recording medium is broadly applied for a facsimile use, a terminal printer of a computer, an automatic bending machine for tickets and a recorder of a measuring instrument, and the use is extending broadly, for example, labels, tickets or slips.
However, along with the extension of uses, requirement for improvement of developed color preservative stability against natural environment such as heat, water or light, body fat when treated by hand, oil, plasticizer when put in a leather purse or solvent and requirement for good ground color are becoming more earnest, and must respond to these requirements.
For example, regarding preservative stability for an image part, it is known that resistance to plasticizer can be improved by using benzyloxalic acid esters compound, bisphenylsulfones compound and a compound possessing epoxy group (refer to patent document 1).
(Patent Document 1)                Japanese Patent Laid-Open Publication 4-164685 (claim)        
Further, recently, the uses for a lottery ticket or a betting ticket of horse race are increasing. In these uses, since there is a possibility that one ticket becomes a bill of high amount, during a long effective term of the bill e.g. from 2 months to 1 year, reliability that the printed letters do not become unreadable or disappear is required. Especially, since these bills are usually kept in common living condition, for example, in cases when these bills are put on dash board of a car and exposed in high temperature atmosphere and ground color is developed or when these bills are wetted by water by spilling of water or hot water or are exposed to rain, it is important that the printed letters has a stability to be readable.
The object of the present invention is to provide a thermally sensitive recording medium which has high recording sensitivity, good heat resistance of ground color and good preservative stability of an image part.